Underground storage tanks are used to store hazardous substances and petroleum products. It is estimated that a significant proportion of the nearly five million tanks in the United States are leaking harmful products into the environment. To ameliorate this problem, the Environmental Protection Agency (the "EPA") has recently promulgated regulations which require that any leakage exceeding a rate of 0.05 gallons per hour be detected and contained.
Methods for detecting leaks in underground storage tanks are well known in the prior art. Most of these techniques use a quantitative approach to identify a leak or to determine leak rate based on a measurement of volumetric changes of the stored product in the tank. The capability of prior art leak detection methods to accurately measure leakage is affected by certain variables such as temperature change, tank deformation, product evaporation, tank geometry and the characteristics of the stored product. In addition to these variables, the presence of ground water around the tank may completely mask an actual leak or at least slow the rate at which the stored product is leaking.
In particular, the water table of the soil in which the tank is buried can vary in height depending of a number of factors including but not limited to location, time of year and amount of rainfall. If the water table is above the location of a hole or break in the tank, the ground water exerts a pressure on that break which counteracts the pressure exerted by the product in the tank. When the water level is above the product level, the pressure exerted by the ground water is greater than the pressure exerted by the product against the break, and thus the ground water will flow into the tank. If the product level is above the water table level, in some cases the pressure exerted by the product will be exactly balanced by the pressure of the ground water at the break and thus leakage out of the tank will be prevented or greatly reduced. In either case, the true nature and scope of the leak cannot be accurately detected.
One way of compensating for ground water "masking" is to simply postpone the leak test until such time as the ground water is below the bottom of the tank. This approach is, of course, highly impractical and expensive. Another approach is to conduct two consecutive tests, each at a different fluid level in the tank, and compare leak rates. The leak rates will differ if there is a leak due to the differeing head pressures. This approach theoretically is independent of ground water levels because the difference in leak rates should be measurable whether ground water is present or not. In practice, the conducting of two separate leak tests is costly and time-consuming. The technique is also unreliable because the differences in leak rate from changes in head pressure are often obscured by temperature-induced volumetric changes.
Accordingly, there is a need for a reliable and economical method for eliminating ground water "masking" effects in a storage tank leak detection system which overcomes these and other problems associated with prior art techniques.